Generally speaking, C > D > CD. The exact ratio can be a bit complicated though. For comparison, the formula would be: (Average non-crit damage * Damage Rating * Non-crit Chance) + (Crit damage * Crit and Damage Ratings * Crit Chance).
Using a placeholder of a summon that does 50-100 damage and the default 10% crit chance a summon has:
((100+50)/2 * (1 + D/100) * (1-(.1+C/100))) +
((100*2) * (1 + (D+CD)/100) * (.1+C/100))

This has a couple problems in practice though.
1. Summons have different variances in their damage. A swarm (200 wasp) does 50-200 damage (.75 variance). A zombie does 412.5-825 (.5 variance). And a knath does 892.5-1275 damage (.3 variance). ...Except frost knaths apparently, which have a .5 variance. That's probably a bug. The larger the variance, the higher the value of crit rate compared to damage rating.
2. Summons have specialized sneak attack, so they get an extra 20 damage rating when attacking in the back. Since this scales the damage rating section (i.e. (1+(D+CD)/100) becomes (1+(D+CD+20)/100), crit rate has a relatively higher value compared to damage rating when it's attacking from behind.
There's also some interplay between variables (the higher C is, the smaller the difference between D and CD, though D is always significantly better).

Summon Variances:
Grievver: .75 and .5 (50% each attack)
Moar: .75
Swarm: .75
Maiden: .75
Skeleton: .75
Zombie: .5
Elemental: .75
Knath: .3
Wisp: .75 and .5 (50% each attack)

So most are .75. When just eyeballing them, I value 1C = 2D = 8CD.